Can you try to play around with the authentication and credentials that are used to call the Powershell script?
It might be a comparable issue to the one state at the end of this article: http://www.vcoportal.de/2013/04/vco-and-veeam-backupreplication-a-powerful-combination/
Also make sure that there are no erros in escaping the commandline parameters. Depending on how you call the powershell script, you might have to do some weird "backslash-wars" to generate the command line properly....
Cheers,
Joerg